Coal with iron content (principally pyrites) concentrated in the heavy (2.9 sink) fraction have shown more tendency to slag than those in which the iron is widely distributed among the various gravity fractions of the coal. The analysis of furnace slags show them to be richer in iron than ash from the coals fired. Iron compounds, particularly the iron sulfides like pyrite (FeS.sub.2) and pyrohtite (FeS) have a much lower melting point and higher specific gravity than most other minerals in coal and coal ash.
Pyrite initially oxidizes to pyrohtite in a boiler flame by the following reaction (1): (1) FeS.sub.2 O.sub.2 .fwdarw.FeS+SO.sub.2. The product FeS forms molten spheres which, due to less aerodynamic drag and higher density (compared to other particles in the furnace fireball), are more likely to mechanically impact and stick on the walls of the furnace. Subsequent chemical reactions between FeS and ash already present on the boiler tubes can form a relatively low-melting slag. Reactions (2) and (3) show the following slag-forming sequence:
(2) 2FeS(s)+30.sub.2 (g).fwdarw.2FeO(s,l)+2SO.sub.2 (g) PA1 (3) FeO(s)+SiO.sub.2 (s,l).fwdarw.FeSiO.sub.3 (s,l) PA1 Code: s=solid; l=liquid; g=gas
Reaction (3) shows the creation of low-melting ferrous silicate slag (2096.degree. F. melting point).
The formation of low-melting slag deposits on furnace water wall tubes is thus greatly enhanced by iron sulfides when they are concentrated in the heavy gravity fraction of a coal.
What is needed is a method and means to grind pyrites or other harmful coal mineral matter to a finely divided state, concentrate them, and insert them into the combustion process at a temperature high enough to quickly convert them to the high melting iron oxides like Fe.sub.2 O.sub.3, or Fe.sub.3 O.sub.4 in the case of pyrites. Converted to this form, the iron compounds and other potentially harmful minerals will not adhere to the furnace wall upon impact, but act as dry collectable ash which will be entrained by the flue gas and therefore is removed from the furnace.